The present invention relates generally to a gas liquid contacting method using a jet venturi scrubber and to apparatus for practicing that method. More particularly, the present invention concerns both an improved gas liquid contacting step by which a scrubbing liquid contacts a low-pressure gas containing pollutants and the apparatus for effecting that gas liquid contact.
The jet scrubber is well suited for use in connection with treatment of low-pressure tail gases since it is capable of handling very large mass flow rates of gas and, at the same time, it does not introduce a large pressure loss in the gas flow. In addition, the liquid can be used to entrain the gas into the liquid and through the venturi at the desired rate. In a jet scrubber, the mass flow rate of liquid is sprayed toward a venturi throat to induce gas flow therethrough. The jet scrubber is thus contrasted to a venturi scrubber which uses an external pump or fan to move gas through its venturi throat. Generally, the gas velocity through the throat of a jet scrubber is relatively low, on the order of 50-60 feet per second.
Jet scrubbers are, of course, known in the art. Oftentimes, however, both a liquid and a gas are injected into the venturi throat in order to pump the secondary flow through the scrubber. In one such device, a nozzle is provided which injects a spray axially toward the throat of the venturi and simultaneously ejects a radially directed generally planar flow across the inlet to the venturi throat so as to create two-staged gas liquid contacting. See, for example, British Patent No. 881,437, published Nov. 1, 1961.
Venturi scrubbers are also known for gas liquid contacting. Conventional venturi scrubbers employ an external mechanical energy source such as a fan to force the gas flow through the venturi itself. Typically, the scrubbing liquid is introduced into the venturi throat in jet-like streams which are atomized by the gas passing through the throat at a relatively high velocity, for example, 200-300 feet per second. These venturi scrubbers have a comparatively high pressure loss when compared with jet scrubbers and, generally speaking, are not desirable for treatment of low-pressure gas streams since additional equipment is necessary.
In any scrubber, it is desirable to obtain a uniform distribution of liquid droplets in the gas flow in order to get uniform gas liquid contacting. In conventional jet scrubbers, it has been observed that a significant radial maldistribution of liquid droplets occurs in the outlet from the venturi throat. This maldistribution manifests itself as a high concentration of liquid in the annular region adjacent to the venturi wall and a low concentration of liquid droplets in the center part of the venturi outlet. Further increases in the liquid mass flow rate are counterproductive in terms of alleviating this observed phenomenon.
These malditribution problems are even further exacerbated when the venturi is scaled up for large processing plants. For example, natural gas processing plants that handle 150,000,000 cubic feet per day of natural gas input are currently under design and construction. In such plants, attempts to scale the jet scrubbers up to throat diameters of 60 inches have demonstrated the impracticality of effecting gas liquid contacting in those scrubbers. The massive maldistribution of the liquid droplets in the jet scrubber outlet causes such insufficient gas liquid contact that the scrubbing effectiveness is materially reduced. For example, it has been observed that reductions in the neighborhood of 75% in the scrubbing efficiency of a jet scrubber can occur when this maldistribution of scrubbing liquid is present.
Accordingly, it will be observed that the need continues to exist for a jet scrubber which is capable of operating at high liquid and gas mass flow rates while retaining its efficacy as a gas liquid scrubber.